A typical optical inspection apparatus consists of a radiation source and a camera composed of an objective and an image plane. The image generated by the objective on the image plane can be inspected and stored e.g. by means of a CCD cell or a CMOS array, which converts the image into an electric signal in a known manner. Such a cell consists of light-sensitive elements disposed as a matrix, e.g. 256×256 elements or pixels. In this case, the properties of the camera resemble those of an ordinary photographing camera. The image plane formed of pixels may also have e.g. the shape 1024×1, and this is then a “linear camera”. This text refers to a camera of the type described above as an electronic camera and it can be used for taking moving pictures or, if desired, also still pictures—in other connections their established name is “video camera” or “digital camera”. Conventional camera optics views the object in different ways depending on the location of the object in the measurement area. At the optical axis of the objective, i.e. the central area of the object, the camera views the object at right angles, and at its edges at an oblique angle, which is larger the greater the distance from the optical axis. This is called the central perspective, which causes detrimental imaging errors for the measurement of the object and quality control in general, and these errors can be corrected by means of telecentric optics. In that case, all the beams from the object arrive in parallel with the optical axis and all the locations of the object are viewed in the same plane perspective. In telecentric objectives, the lens or concave mirror closest to the object should have a width equalling at least the object, and this results in heavy and bulky optical equipment consisting of ordinary lenses and/or mirrors. EP 1 089 106 discloses a relatively light and simple solution to these problems. The telecentric design of this reference uses a strip-like planar parabolic mirror, the aperture of the objective proper being located in the area of the focal plane of the mirror. This objective proper, in turn, is integrated in a non-telecentric camera, which generates an image on a light-sensitive image plane.
In many cases, it is necessary to measure and/or inspect the object also from other directions than one specific direction. Thus, for instance, it may be necessary to monitor sawn timber from the direction of two opposite faces or from the directions of all four faces. This can obviously be done by means of four devices directed towards the upper surface, lower surface and lateral surfaces of the object, but this incurs high equipment costs. Another option is turning the object and running it through the imaging area of a telecentric imaging unit four times. Firstly, such an arrangement is slow in terms of production and secondly, mutual positioning of the image data obtained on different sides of the object is problematic. WO 94/24516 depicts an arrangement for measuring the width of a moving object by using two parabolic mirrors in connection with one camera, together with an elongated light source providing background light. For measuring the thickness of the object, laser included in the arrangement and a second camera are used, and if necessary, a second laser and a third camera. This arrangement only allows for measurement of the boundary dimensions of the object in different projections.
DE 41 04 501, again, explains an arrangement for determining the sapwood side and the heartwood side of two even surfaces of sawn timber, such as planks and boards. The reference makes a difference between these opposite sides by utilising their different grain densities, i.e. growth ring densities. In order to determine these different grain densities, the reference suggests passing the timber body between at least one pair of sensors, with the sensors disposed opposite each other. These at least two sensors consist of a transmitter and a receiver operating in the range of visible light or infrared light. The sensors identify the growth rings on the basis of the different light reflectivity of adjacent locations on the timber body, and these initial data of the opposite sensors regarding differences in reflection density are fed into a comparator in the reference, the comparator calculating by means of not represented software which of the two opposite sides is sapwood and heartwood, respectively. The inherent structure of the sensors has not been described in any way, however, the figures of the reference and the definition “transceiver” allow the conclusion that the sensor detects only one pixel at a time, without any optics proper. The reference does not specify whether the measurement is based on average reflection densities of larger areas of the object—in the case of a large-sized pixel—or on the reflection density provided by transversely movable sensors or a plurality of sensors—in the case of a small-sized pixel. In other words, the arrangement of this reference allows observation of two surfaces of the object, however, the number of sensors is at least equal to the number of inspected surfaces. The reference does not indicate the manner of inspecting the entire area of even one surface of the object.